Polyamide fine powders are used in numerous cosmetic products, such as, for example, in lipsticks, make-ups, powders and creams. They generally comprise fine powders based on polyamide 11 and/or polyamide 12, and also polyamide 6 and PA6/12 copolymers having an average particle diameter of from 5 to 30 μm. Polyamide powders with a larger average particle diameter of up to 400 μm are used, inter alia, in shower gels to achieve a peeling effect.
Since cosmetics are in direct contact with the human skin, high dermatological requirements are placed on their contents with regard to purity and their skin compatibility. Polyamide fine powders used hitherto already satisfy the high purity requirements. The precipitation process and anionic polymerization, in particular, are known for preparing such cosmetic powders based on polyamide. Other specific processes preparing cosmetic powders, as disclosed in JP 04050232 and JP 05070598 (both Daicel-Hüls) and JP 05025019 (Kao Corp.), have not achieved industrial importance.
However, powders from both processes frequently have, as a result of their respective preparation process, a pH which is outside the range which is regarded by dermatologists as the optimum pH range for the acid protective mantle of the skin. A pH which is either too acidic or too basic can lead to skin irritations. In particular, a basic pH also causes the skin to dry out. As a result, the skin becomes brittle and its elasticity decreases. In addition, under stress, cracks and small skin injuries may rapidly result.
The precipitation process described in DE 29 06 647 B1, provides a suitable polyamide-12 granulate, which is completely dissolved in hot alcohol under pressure. Upon cooling fine particles crystallize out, the growth of which is controlled by the cooling rate and optionally also by small amounts of phosphorus-containing acids. When crystallization of the polyamide particles is complete, the alcohol is stripped off. Small amounts of phosphorus-containing acids (e.g. hypophosphorus acid, phosphorus acid and/or phosphoric acid), however, remain incorporated within the polyamide, as a result of which these fine powders tend to have a slightly acidic reaction in aqueous suspension, the pH being 4-6 depending on the carboxyl end-group excess and phosphoric acid content, and in extreme cases may even be below pH 4. These pH values do not correspond to the market requirements in certain fields.
During anionic solution polymerization, an anionically activated polymerization of monomeric lactam, preferably laurolactam, takes place in higher-boiling paraffins (140 to 170° C.). Strongly basic catalysts are used for this purpose, as a result of which such fine powders usually have a slightly basic/alkaline reaction despite repeated washing. The pH values of such powders are between 7 and 8.5.
The fine powders obtained from both processes have a round particle shape. Powders from the two processes are usually protection-screened and/or sifted and may optionally be subsequently ground to increase the proportion of fines.
Most cosmetic preparations generally comprise only between 0.1 and 10% by weight of polyamide fine powder. Depending on the field of use, the remainder consists of a variety of oils, fats, emulsifiers, stabilizers, and also dyes and aroma substances. They usually comprise a sufficiently large buffer system in order to produce a cosmetic application with a skin-friendly pH.
However, for some time there has also been an explicit desire on the part of the cosmetics industry for pH-regulated polyamide powders.
Surprisingly, it has now been found that the pH of polyamide powders as described in the claims, in particular precipitated powders, polyamide powders which have been prepared according to the precipitation process, can be well regulated if they are equipped with a special buffer system. This buffer system is based here preferably on naturally occurring acids and their corresponding salts. In addition, the slightly basic polyamide powder can also be buffered using the process according to the invention.